Biology Reference
In-Depth Information
a variety of purposes ranging from locomotion (Chapters 7 and 8) to sensation to fertilization,
and many of these are based on microfilament skeletons that are orientated with their pointed
ends towards the main body of the cell and their barbed ends towards the tip of the process.
Thyone briareus
(syn:
Holothuria briareusi,
sea cucumber) is a type of marine echinoderm
that lives in mud and is abundant at the famous Woods Hole laboratory, where it has been
studied extensively. Its eggs are covered in thick, protective jelly that creates a problem for
sperm attempting to fertilize them. The problem is solved in two main ways. First, the
head of the sperm bears an acrosomal vesicle that is full of enzymes capable of digesting
the jelly coat, and these enzymes are released when receptors on the sperm detect that
they have bound the coat of an egg. Second, the sperm produces a long, rigid process, the
acrosomal process, that forces its way through the digesting jelly coat to meet and fuse
with the surface of the egg. Formation of this acrosomal process is rapid and dramatic; it
extends from nothing to become 90
m
m long within a mere 10 seconds.
50,51
It is constructed
mainly from microfilaments.
Before binding, actin is present at high concentrations near the acrosome but it is com-
plexed with profilin and other proteins. A small amount of the actin is already polymerized
into a bundle of about 25 microfilaments, the actomere,
52
arranged with their barbed ends
facing the membrane. Once sperm-egg binding has taken place, actin polymerization
proceeds from the actomere extremely quickly (
Figure 5.12
). Profilin skews the efficiency
of the binding of actin monomers to barbed and pointed ends so that only the barbed
ends grow despite the high concentration of actin present. This can be shown by adding
either actin or actin-profilin complexes to an acrosomal bundle
in vitro
: actin alone will
extend it from both ends (demonstrating that neither end is blocked by capping proteins),
whereas actin-profilin extends it almost exclusively from the barbed end.
53
The direction of actin polymerization in the acrosomal processes of
T. briareus
creates two
potential problems for the cell: restricted transport of unpolymerized actin to the growing tip,
and restricted access to filaments even when actin has got there.
Actin monomers are added to the distal end of the growing process, which gets further
and further from the main body of the cell as polymerization proceeds. This creates an
FIGURE 5.12
Growth of the acrosomal process in sperm of the sea cucumber
Thyone briareus.
